Posttranscriptional control of gene expression plays a key role in regulating gene expression in cells undergoing apoptosis. Cyclooxygenase-2 (COX-2) is a crucial enzyme in the conversion of arachidonic acid to prostaglandin E2 (PGE(2)) and is significantly upregulated in many types of adenocarcinomas. COX-2 overexpression leads to increased PGE(2) production, resulting in increased cellular proliferation. PGE(2) enhances the resistance of cells to ionizing radiation. Accordingly, understanding mechanisms regulating COX-2 expression may lead to important therapeutic advances. Besides transcriptional control, COX-2 expression is significantly regulated by mRNA stability and translation. We have previously demonstrated that RNA binding protein CUGBP2 binds AU-rich sequences to regulate COX-2 mRNA translation. In the current study, we have determined that expression of both COX-2 mRNA and CUGBP2 mRNA are induced in MCF-7 cells, a breast cancer cell line, following exposure to 12 Gy gamma-irradiation. However, only CUGBP2 protein is induced, but COX-2 protein levels were not altered. Silencer RNA (siRNA)-mediated inhibition of CUGBP2 reversed the block in COX-2 protein expression. Furthermore, MCF-7 cells underwent apoptosis in response to radiation injury, which was also reversed by CUGBP2 siRNAs. These data suggest that CUGBP2 is a critical regulator of the apoptotic response to genotoxic injury in breast cancer cells.

Damage to intestinal epithelium limits the use of ionizing radiation (IR) in cancer therapy. Prostaglandins (PGs), generated through the action of cyclooxygenase-1 (COX-1) and COX-2 protect the intestinal stem cells from IR. In previous studies, we demonstrated that the RNA-binding protein CUGBP2 regulates the stability and translation of COX-2 mRNA by interacting with AU-rich sequences in 3 UTR. Here, we demonstrate a dynamic antagonistic relationship between CUGBP2 and COX-2. Both CUGBP2 and COX-2 are rapidly induced after IR in intestinal crypt epithelial cells in mice, but CUGBP2 protein expression is observed immediately and COX-2 protein expression is delayed. In contrast, administration of bacterial lipopolysaccharide induced COX-2 expression and PGE 2, resulting in the inhibition of CUGBP2 expression and radioprotection of the intestine. These effects were reversed by NS398, a COX-2-specific inhibitor, suggesting that lipopolysaccharide-mediated inhibition of CUGBP2 is a PG-dependent mechanism. Furthermore, CUGBP2 expression is higher in COX-1 ؊/؊ and COX-2 ؊/؊ mice than wild-type controls at basal conditions, which is further increased after IR.

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